This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the
pathway.
If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.

L-ascorbate, also known as vitamin C, fulfils multiple essential roles in both plants and animals. Being a strong reducing agent, it functions as an antioxidant and a redox buffer. It is also a cofactor for several enzymes, which are involved in many important pathways, including collagen hydroxylation, carnitine biosynthesis, norepinephrine biosynthesis, and hormone and tyrosine metabolism. In plants L-ascorbate is also implicated in defense against pathogens and in control of plant growth and development. A significant proportion of a plant's ascorbate is found in the apoplast (the aqueous solution permeating the cell walls) [Green05].

2,3-dioxo-L-gulonate (DKG) is a well established degradation product of L-dehydro-ascorbate (DHA), although the pathway(s) it is involved in are not well characterized. A study of ascorbate degradation by human lens homogenates found that under non-oxidative conditions DHA was hydrolyzed to DKG, which was subsequently converted to oxalate and L-erythrulose [Simpson00]. These findings contradicted an earlier study of L-ascorbate degradation into oxalate in several plants, which found that DKG was a very poor substrate for oxalate production (although it is possible that the pathways differ among plants and animals) [Yang75].

This pathway describes the findings of a careful study that evaluated the different chemical reactions involved in DHA catabolism in the plant apoplast [Parsons11]. The study determined that DHA is a branch point in ascorbate catabolism - under oxidative conditions it is oxidized to oxalate and its esters (see L-ascorbate degradation III), while under non-oxidative conditions it is hydrolyzed to DKG. The oxidation:hydrolysis ratio is determined by the concentration of reactive oxygen species.

Under non-enzymatic, non-oxidative conditions, DKG was formed slowly and converted to 2-carboxy-L-xylonolactone, which was reversibly de-lactonized to 2-carboxy-L-threo-pentonate [Parsons11]. There was little evidence for enzyme activity in the catabolism of DKG. The presence of either L-ascorbate or hydrogen peroxide supressed the formation of 2-carboxy-L-xylonolactone, suggesting that DKG was diverted under these conditions to competing oxidative pathways.

The fact that the pathway proceeds non-enzymatically suggests that it may occur in the apoplast of most, if not all plants.